先化简，再求值：（1﹣$\frac{x}{x+1}$）÷$\frac{x^2-2x+1}{x^2-1}$，其中x=$\sqrt{5}$+1．

依据“站得高看得远”的道理，我们应站在下列什么地方才能看得最远

①命题“∀x∈R，x³﹣x²+1≤0”的否定是“ $\exists x_0\in R,x_0^3-x_0^2+1>0$ ；

②“ $b=\sqrt{ac}$ ”是“三个数a，b，c成等比数列”的充要条件；

③“m=﹣1”是“直线mx+（2m﹣1）y+1=0和直线3x+my+2=0垂直”的充要条件：

    Many teachers believe that hands－on experience is the ____(good) way to learn. A group of students in the United States are putting that belief to the test. They ____ (success) designed and built a robot for discovering unexplored mines ____ the bottom of the sea. Students made the robot ____(answer) a challenge from the U．S. Department of Defense.

    The students recently tested the underwater vehicle ____(call) PerseusⅡin a 95－meter－long tank of water at the Stevens Institute of Technology.

    The robot costs about $15,000 to build. The students use a video game control to direct ____(it) movements in the tank. What's more, devices known as Japters ____ (able) the robot to go up, down and toward its target in the water. Video cameras on the robot send images back to a computer through a 13－meter－long cable. Perseus Ⅱ,____the students worked up to 20 hours a week to design and build, also has ____ set of lasers (激光), which are used to measure the size of an object.

    DeLorme supervised the project from ____(begin) to end. He says the specialized requirements of robots were demanding.